Phyllotungstite

Chemical formula: HCaFe<sup>3+</sup><sub>3</sub>(W<sup>6+</sup>O<sub>4</sub>)<sub>6</sub>·10H<sub>2</sub>O

Phyllotungstite is a very rare, secondary tungsten mineral, forming microscopic, yellow, platy crystals.

## Characteristics Phyllotungstite is a hydrated calcium and iron tungstate, belonging to the secondary minerals. It occurs as extremely small, thin, platy crystals with a hexagonal outline, which typically form rosette-like or spherical aggregates, as well as coatings and efflorescences. Due to the microscopic size of the crystals, its macroscopic features are difficult to observe without adequate magnification. ## Physical Properties Phyllotungstite crystals exhibit a luster described as pearly to submetallic. They are flexible, which is an unusual characteristic for minerals. Hardness and density have not been precisely measured due to the small size and rarity of samples. ## Colors and Varieties This mineral has a characteristic, bright yellow color. No varieties have been distinguished. ## History and Name Phyllotungstite was first described in 1984 by J. E. J. Martini. Its name refers to its crystal structure and chemical composition. The Greek word *phyllon* means "leaf," referring to its lamellar structure, and the component *tungstite* indicates the presence of tungsten and a visual resemblance to wolframite. It was approved by the International Mineralogical Association (IMA) under number IMA1983-028. ## Uses Phyllotungstite has no industrial application. Its significance is purely scientific and as a collector's item, being a very rare secondary mineral from the oxidation zones of tungsten deposits.

Properties

Mohs hardness
2
Luster
Pearly
Streak
Pale yellow
Density
4.35
Cleavage
Perfect on {0001}
Fracture
Micaceous
Transparency
Translucent
Crystal system
Hexagonal

Diagnostic features

## Identification Phyllotungstite is identified based on its characteristic appearance: microscopic, yellow, hexagonal plates forming rosette-like aggregates. Its occurrence in the oxidation zones of tungsten deposits, often in association with scheelite or ferberite, is key. Final identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Differentiation from Similar Minerals It can be confused with other secondary yellow tungsten minerals, such as ferritungstite or tungstite (WO₃·H₂O). It differs from tungstite by the presence of calcium and iron in its composition and its hexagonal crystal form (tungstite crystallizes in the orthorhombic system). Differentiation from ferritungstite is very difficult without chemical analysis. ## Crystal Forms It forms very thin, platy crystals with a hexagonal outline. These crystals aggregate into forms resembling rosettes, spherulites, or fans, and also create thin coatings on other minerals.

Geological environment

## Genesis Phyllotungstite is a secondary mineral, forming in the oxidation (weathering) zones of tungsten deposits. It forms as a result of the alteration of primary tungstate minerals, such as scheelite (CaWO₄) and ferberite (FeWO₄), under low temperature and pressure conditions, with access to oxygen and water. ## Mineral Associations It most commonly co-occurs with the minerals from which it forms, namely scheelite and ferberite. It is also accompanied by other minerals from the oxidation zone, such as quartz, goethite, hematite, and other rare tungstates. ## Localities The most important and type locality (località tipo) for phyllotungstite is the tungsten mine in Gifurwe, Northern Province, Rwanda. This is the only confirmed and well-documented locality for this mineral worldwide.

Rarity

Very rare

For collectors

## Quality Criteria The quality of phyllotungstite specimens is primarily assessed based on the richness and development of the crystal aggregates. The most prized samples are those with clearly formed, undamaged rosettes of intense yellow color, contrasting well with the host rock (matrix). Due to their microscopic nature, specimens are evaluated under magnification. The size of the area covered by the mineral is also important. ## Popular Localities The only source of phyllotungstite specimens is its type locality – the Gifurwe mine in Rwanda. All samples available on the collector's market originate from this single location.

Care and storage

## Cleaning Due to the extreme delicacy and small size of the crystals, mechanical cleaning is inadvisable. If necessary, compressed air (from a safe distance) can be used to remove dust. Contact with water and any chemicals should be avoided. ## What to Avoid Contact with water, acids, and other cleaning agents must be strictly avoided. Specimens are very brittle and sensitive to shocks and abrasion. They should not be exposed to high temperatures or direct sunlight. ## Storage Phyllotungstite specimens should be stored under stable conditions, in closed, padded "micromount" boxes to protect them from mechanical damage, dust, and moisture. It is best to keep them in a dry place.

Sources

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